In recent years, a high-density optical disc such as a Blu-ray Disc (registered trademark) has been proposed. In the high-density optical disc, although it has the same shape as that of a DVD (Digital Versatile Disc), it can have a recording capacity of about 25 Gigabytes for a one-side single layer and about 50 Gigabytes for one-side double layers.
In the high-density optical disc, by setting a wavelength of a light source to 405 nm and setting a numerical aperture NA of an objective lens to a large value of 0.85, a diameter of a beam spot for recording and reproducing is reduced to 0.58 μm, thereby enabling a spot area which is equal to about ⅕ of that of the DVD to be obtained.
By increasing the numerical aperture NA of the objective lens as mentioned above, an angle error (referred to as a tilt margin) which is permitted in an inclination from 90° as an angle defined by a disc surface and an optical axis of a laser beam decreases.
In the high-density optical disc of the one-side single layer, therefore, a cover layer which covers an information layer is thinned to a thickness of 0.1 mm. In the case of the high-density optical disc of the one-side double layers, an information layer serving as a reference layer (the 0th recording layer) is formed at a position of a depth of 0.1 mm (100 μm) when seen from the incident direction of the laser beam and an information layer serving as an additional layer (the first recording layer) is formed at a position of a depth of 75 μm. The information layer denotes both of a reflecting layer in a read only optical disc and a recording layer in a writable optical disc. Hereinbelow, the 0th recording layer is properly referred to as an L0 layer and the first recording layer is properly referred to as an L1 layer.
To realize the further larger capacity as mentioned above, the optical disc of a multilayer structure having a plurality of information layers is considered to be promising.
Hitherto, as a manufacturing method of the read only high-density optical disc of the one-side double-layer structure, the following method has been known. When a substrate is molded, pits of the L1 layer are transferred by using a resin stamper for transfer having light transmitting performance constructed in such a manner that pits of the L0 layer have been formed onto one principal plane, an intermediate layer made by an ultraviolet (UV) hardening type resin has been formed onto one principal plane of a substrate formed with the pits of the L0 layer, and concave/convex portions to form the pits of the L1 layer have been formed on the surface of the intermediate layer. The ultraviolet rays are irradiated through the resin stamper. The UV hardening type resin is hardened. Thereafter, the resin stamper is peeled off and the pits of the L1 layer are formed.
For example, a technique in which the reasonable and light-weighted resin stamper can be obtained by using polycarbonate as a material of the resin stamper has been disclosed in JP-A-2002-260307.
However, the manufacturing method of the optical disc as mentioned above has the following problems. The resin stamper made of polycarbonate has such a problem that peel-off performance for the intermediate layer is not so good. Although a cycloolefin system resin material whose peel-off performance for the intermediate layer is good can be used as a material of the stamper, there are such problems that it is expensive, if it is used a plurality of times, its stickness is enhanced, the peel-off performance deteriorates, and the like. Further, according to the resin stamper, since the resin material deteriorates or is deformed by the irradiation of the ultraviolet rays, or the like, it is difficult to use it for the transfer of plural times. Therefore, ordinarily, the resin stamper is thrown away every transfer. There are, consequently, such problems that the material is wastefully consumed and the disposing process is burdened.
In the high-density optical disc as mentioned above, for example, it is necessary to set a thickness of substrate to about 1.1 mm. In the case where such a thick substrate is molded by injection molding, there are such problems that it takes a time to cool the substrate after the molding and a manufacturing cycle time of the optical disc is long.
In the optical disc in which the protecting layer having a thickness of about 0.1 mm or the like is formed on the substrate having a thickness of 1.1 mm and the thickness structure is unbalanced, for example, like a high-density optical disc as mentioned above, there is such a problem that a change such as a warp or the like is liable to occur in the disc due to moisture absorption or the like.
It is, therefore, an object of the invention to provide a manufacturing method of an optical disc whereby good information layers can be efficiently and reasonably formed without wastefully using a resin stamper and, further, a shape change such as a warp or the like can be suppressed and to provide the optical disc manufactured by the manufacturing method.